This invention relates generally to brake valves and accompanying brake systems, and more particularly concerns a valve having both power and manual modes for use in large or complex hydraulic brake systems.
Brake control valves such as those described in U.S. Pat. Nos. 3,827,765 and 3,900,229 are increasingly used for actuating brake systems of relatively large size or relatively complex nature. Brake systems such as these are especially attractive when offered on off-highway equipment such as farm tractors, mining equipment, front-end loaders, cranes, road rollers, and the like. The actuator valves for these brake systems commonly can be operated in either a "power" mode or a manually actuated mode.
It is important to operators of such equipment that the brake systems on their equipment provide powerful but smooth pressure modulation in order that braking can be applied with maximum accuracy and predictability. Good modulation means good "feel" at the brake pedal, and good "feel" permits the experienced operator to obtain maximum performance from his equipment.
When the brake system is being operated in its power mode, brake fluid is pressurized and then supplied to the brake units from a pressurized fluid source such as a pump. The power valve acts as a brake fluid pressure and flow modulator.
When the brake system is being operated in the manual mode, the power valve acts as both a fluid pressurizer and as a fluid pressure modulator. To pressurize and modulate, valves such as those described in the U.S. Pat. Nos. 3,827,765 and 3,900,229 patents include a bore in which slides a piston, forming a collapsible chamber. When the piston is moved by a connected brake foot pedal or other actuator, fluid is pressurized and delivered to the brake units to cause braking action.
Until now, the brake design engineer has been faced with something of a dilemma in selecting a brake valve with desirable manual mode pressurizing and modulating characteristics. A large-diameter valve piston and correspondingly large diameter fluid pressurizing collapsible chamber can be provided, in order that only short brake pedal travel be necessary to displace relatively large volumes of fluid. These large-volume fluid charges quickly take up clearances in the brake units and move the brake shoes or pads into contact with the mating brake surfaces. However, relatively large pedal forces are then required to apply adequate braking pressure to the fluid and brake units through the valve.
On the other hand, a relatively small diameter piston and correspondingly small diameter collapsible valve chamber provide the high brake pressure desirable for powerful braking action, but relatively long pedal movement or stroke is required if the valve is used with high fluid volume brake units or large braking systems.
It is the general object of the present invention to provide the advantages of both a large-piston, large-cylinder and a small-piston, small-cylinder brake valve in but a single unit, while eliminating the described disadvantages of both the small-piston and large-piston unit.
Another object is to provide a brake system valve of the type described which is reliable and rugged in operation, and which will provide a relatively long service life without extensive maintenance requirements.
Yet another object is to provide a brake valve of the type described which can be offered at a commercially attractive price.
A further object is to provide a brake system which utilizes the features of this valve to maximum advantage. An related object is to provide such a system in which many system components can be carried in or upon the valve itself for ease of servicing.